An integral part of bipolar and BiCMOS integrated circuits is an epitaxial layer which serves as the collector and provides a highly ordered crystalline layer into which subsequent diffusions and implants of dopants can be made during the device fabrication. A buried collector is diffused or implanted into patterned areas of the substrate before the epitaxial layer is formed. For fabrication of NPN transistors, lightly-doped n-type (or intrinsic) epitaxial silicon is deposited above the heavily doped n-type buried layer (sub-collector). The dopant concentration in the buried layer is typically about 3-5 orders of magnitude higher than in the epitaxial layer.
In the limiting case of RF circuits for BiCMOS technologies, isolation schemes are required that provide reduced collector-substrate capacitances for NPN bipolar junction transistors (BJTs) while supporting high packing density for MOS and bipolar junction transistors. The use of buried collectors with highly doped shallow junctions and reduced lateral diffusion allow lower isolation doses to support minimum device spacings. This reduced peripheral area of the buried layer results in a reduction in the peripheral component of the collector-substrate capacitance which is typically dominant in NPN transistors. In order to achieve formation of abrupt, low-resistance buried collectors, epitaxial deposition processes are needed which are capable of depositing thin (&lt;2 um) films without significant diffusion of the dopant occurring during the deposition processes either into the substrate or into the forming epitaxial layer. It is also important to minimize loss of the buried layer dopant through evaporation during the epitaxial deposition process and to minimize the reincorporation of evaporated dopant (referred to as autodoping) either vertical or lateral to the buried layer.
Because of its higher solid solubility, arsenic (As) is the preferred dopant for the formation of the buried collector profile. However, due to a high sticking coefficient, arsenic has the disadvantage of making the epitaxial film growth process very susceptible to autodoping. Vertical autodoping effects the intrinsic collector doping and decreases collector-base breakdown voltages. Lateral autodoping compensates isolation autodoping and, in turn, degrades the trade-off between peripheral substrate capacitance and device packing density.
Conventional methods for forming an epitaxial layer use a high temperature (1150-1200.degree. C.) pre-bake in a hydrogen ambient before epitaxial deposition. The high temperature pre-bake removes the native oxide from the surface and causes dopants from the buried layer to outgas and evaporate into the gas ambient above the wafer surface or become mobile atoms absorbed to the wafer surface. Although lower temperatures can be used to effectively rid the surface of native oxide, the sticking coefficient if the arsenic atoms increases with decreasing temperature and can result in severe vertical and lateral autodoping profiles. For this reason, a high temperature pre-bake is typically used to remove the surface oxide and evaporate some of the arsenic from the surface while it exhibits lower propensity to stick to the surface. Low pressure is used during the pre-bake to draw the outgassed dopants away from the wafer surface and out of the reactor to exhaust. A thin epitaxial layer may next be deposited to cap the buried layer followed by a flush step to reduce the dopant concentration in the reactor after the dopant source has been capped. The temperature may then be further lowered (to minimize diffusion of the buried collector) to deposit the remaining portion of the epitaxial layer. The high temperatures involved in order to minimize autodoping cause significant amounts to outgas from the buried layers. Thus, the buried layers must be more heavily doped initially to compensate for the loss during the epitaxial pre-bake steps. This, in turn, allows more diffusion of the buried layer in the substrate consequently increasing collector-substrate capacitance and also requires higher temperature anneals to remove implant damage. A method that further reduces the amount of updiffusion and lateral autodoping is needed.